1. Field of the Invention
The present invention relates to discrete multitone (DMT) based digital subscriber line (DSL) transmission systems which allow high speed communication on twisted pair telephone lines. The invention relates more specifically to a VDSL (Very high speed DSL) system which can be used with several existing or forthcoming standards.
2. Discussion of the Related Art
FIG. 1 shows the spectrum of a signal transmitted according to the ADSL and ADSL-Lite (asymmetric DSL) standards. The ADSL standard uses quadrature amplitude modulation (QAM) on each of 256 tones, the tones being equally spaced by 4.3125 KHz. Thus, as shown, the last tone has a frequency of 1.104 MHz. The ADSL-Lite standard only uses the first 128 tones.
As shown, a gap is left at the beginning of the spectrum for “plain old telephone services” (POTS).
According to the ADSL standards, most of the tones are used for reception, the few remaining tones being used for transmission, hence the term “asymmetric DSL”.
Current VDSL standardization proposals contemplate the use of frequencies up to 11.04 MHz.
FIG. 2 shows the spectrum of a signal transmitted by a conventional VDSL time domain duplexing (TDD) system such as described in “VDSL Alliance SDMT VDSL Draft Standard Proposal”, ETSI STC/TM6, 973T13R0, Lannion, France, September 29–Oct. 3, 1997. This system uses 256 or 512 tones spaced, respectively, by 43 or 21.5 KHz. The last tone has a frequency of 11.04 MHz. All the tones are used for a same transmission direction at one time, the transmission direction being switched every other transmitted symbol.
FIG. 3 shows the spectrum a signal transmitted by a conventional VDSL “Zipper” system as disclosed in patent application WO 97/06619. It uses 2048 tones spaced by 5.375 KHz, the last tone also having a frequency of 11.04 MHz. In this system, the tones used for transmission and for reception are chosen dynamically in order to cancel near-end crosstalk and near-end echoes.
FIG. 4 very schematically shows a DSL transmission system at one end of a telephone line 10. An inverse fast Fourier transform (IFFT) circuit 12 receives N complex frequency domain coefficients, where N is the number of tones used by the system, i.e. 128 or 256 for the ADSL standards, 256 or 512 for the VDSL TDD system, and 2048 for the VDSL Zipper system. The IFFT circuit 12 generates, for each set of N coefficients, a time domain symbol. A symbol is thus the sum of N sinusoidal subcarriers of different frequencies corresponding respectively to the tones. The amplitude and phase of each subcarrier is determined by the corresponding frequency domain coefficient received by the IFFT circuit. The symbols are processed by a digital-to-analog converter 14 and a low-pass filter 16 and then transferred onto telephone line 10 through a hybrid line interface 18.
A cyclic prefix and a cyclic suffix are added to the symbol output by IFFT circuit 12 at 19. The cyclic prefixes are intended to eliminate intersymbol interference in the far-end receiver by providing a guard period during which the propagation transients of the line may decay. The cyclic suffix is intended to cancel the effects of the sampling of discontinuities in near-end echoes.
Line interface 18 also receives incoming symbols from line 10. These incoming symbols are provided to a fast Fourier transform (FFT) circuit 20 through a low-pass filter 22, an analog-to-digital converter 24 and, if necessary, through a time domain equalizer 26.
The above mentioned cyclic prefix, in order to accomplish its role, has a minimum length independent of the symbol length. In DSL systems using a relatively low number of tones, such as ADSL and VDSL TDD, the transmitted symbols are short, whereby the minimum length of the cyclic prefix is so long that it causes a substantial efficiency loss in the data transmission. In this case, the cyclic prefix is chosen shorter than necessary and it is the role of the time domain equalizer 26 to complement the short cyclic prefixes in the elimination of the intersymbol interference.
In DSL systems using a large number of tones, such as in the VDSL Zipper system, the generated symbols are so long that the cyclic prefixes can be chosen at the necessary length without substantially affecting the efficiency of the transmission. In such systems, the time domain equalizer 26 is omitted.
Moreover, in a VDSL TDD system, since the IFFT circuit and FFT circuit are never used at the same time, it is a single circuit which performs both functions.
The IFFT and FFT circuits operate at least at twice the frequency of the last tone used by the system, i.e. 1.104 MHz for ADSL-Lite, 2.208 MHz for ADSL, and 22.08 MHz for the known VDSL systems.
It is clear that the ADSL standards and forthcoming VDSL standards differ in many ways (the number of used tones, the spacing between the tones, the operation frequency of the IFFT and FFT circuits . . . ), which is likely to increase the number of types of modems capable of exploiting these standards.